I. Field of the Invention
This invention relates to processes and apparatus for heating bodies at high temperature and pressure. More particularly, the invention relates to such processes for heating bodies made of ceramics and the like utilizing microwave energy to bring about the desired heating.
II. Discussion of the Prior Art
Sintered shaped bodies are often formed by heating shaped pre-forms made of ceramic powders, e.g. alumina, to the required sintering temperature normally in the range of 1000.degree.-2000.degree. C. In conventional sintering, the pre-form to be sintered is brought up to the required temperature by placing it in a gas fired furnace or a furnace equipped with resistant heating elements. The product is heated by radiation and the thermal energy thus generated has to penetrate into the pre-form from the surface. In order to produce a crack-free product, the thermal stresses imposed by the internal temperature gradients have to be minimized. Mainly for this reason, but also in order to minimize thermal stresses within the furnace elements, the temperature of the product has to be increased slowly, and furnace cycle times of 24-48 hours are not uncommon. Because a significant amount of furnace insulation and wall mass is also heated up with the product, the thermal inertia of the system is quite large and rapid control of the temperature is not possible. Finally, element failure is quite common in high temperature electrical furnaces especially where temperature cycling occurs.
One way of improving the quality of sintered ceramic products is to use a hot isostatic press. In such a press, the product is introduced into a pressure vessel which is then sealed. Heating elements within the pressure vessel are energized and the product is heated. At the appropriate time, a gas at high pressure (typically 20,000 psi or higher) is introduced into the pressure vessel. The high pressure acts to compress the product uniformly in all directions and helps to correct any defects that may have formed during the heating. Thus, in general, hot isostatic pressing involves the following two stages:
(1) the pre-form is sintered until all open porosity is eliminated; and PA0 (2) the pre-form is then subjected to high isostatic pressure.
Normally, for effective utilization of a hot isostatic press, sintering and hot pressing are done in two separate furnaces. As a result, the system is an expensive one and can be used only for extremely high value added products.
Sintering of ceramic powders by means of microwave heating is disclosed in Japanese Patent Publication 60-221367 dated Nov. 6, 1985 in the name of Nippon Microwave KK. This procedure involves placing ceramic powder in a ceramic receptacle located in a microwave cavity resonator or in a waveguide. Microwaves are guided into the resonator or waveguide while the powder is uniaxially pressed by means of a mechanical die or piston. However, uniaxial compression of this kind does not produce the desirable effects achieved by isostatic pressure.